tilde (~) directories in Perl - perl

Question

I found a slight misbehaviour in my Perl script when I create and check for the existence of directories with a tilde sign, which doesn't happen if I use a full /home/user path. When I run this script for the first time, it creates the new directory. When I run it the second time, it doesn't recognise the existence of the directory, and tries to create it a second time:
#!/usr/bin/perl
use strict;
my $outdir = '~/test';
my $cmd = "mkdir $outdir";
unless (-d $outdir) {
0 == system($cmd) or die "Error creating outdir $outdir\n $?";
}
1;
[~] $ rm test/ -rf
[~] $ perl dir.pl
[~] $ perl dir.pl
mkdir: cannot create directory `/home/avilella/test': File exists
Error creating outdir ~/test
256 at dir.pl line 7.
How can I reliably deal with directories that use the tilde ~ sign in Perl?

Answer 1

The tilde is interpreted by the shell to mean your home directory.
Hence Perl's -d operator sees something different (a file/directory called ~) to your shell invocation 'mkdir ~/whatever' (which expands ~ to mean /home/user).
I would try to use exclusively Perl functions to perform your operations. You'll avoid spawning new processes and your file access will be performed in a consistent fashion.
Note Perl's mkdir built-in function. Note also the File::Glob module which does perform expansion of the ~ character (perhaps useful if you have users entering directory names manually)

Answer 2

You can use the %ENV home directory, which is the values imported from the shell:
my $home = $ENV{HOME};
You should also know that mkdir is a Perl built-in function:
mkdir "$home/test" or die "Cannot create test: $!";

Answer 3

~ is interpreted by the shell that is invoked by the system function. It's the shell that replaces ~ by the user's home directory. As far as Perl or the kernel is concerned, ~ means a file or directory with a one-character name, like any other character. So the test done by -d fails, because there's no directory called ~.
If you'd used Perl's built-in mkdir function rather than calling an external command via a shell script, you would have had an error at that point because the directory ~ doesn't exist.
The user's home directory is almost always available in the environment variable HOME. If you like, you can fall back to querying the user database if HOME is not present, but that's an abnormal situation. Do use the HOME environment variable if it is present, because it is sometimes useful to change it to run a program with different configuration files, and the environment variable is always available in practice whereas the user database could be unavailable due to network trouble in some configurations (e.g. NIS or LDAP).
my $home_directory = $ENV{HOME};
if (!defined $home_directory) {$home_directory = getpwuid($<);}
my $outdir = "$home_directory/test";
unless (-d $outdir) {
mkdir $outdir or die "Error creating $outdir: $!\n"
}

Answer 4

Your script can't create directories which exist. That's the error you presented us:
[~] $ rm test/ -rf
[~] $ perl dir.pl
[~] $ perl dir.pl
mkdir: cannot create directory `/home/avilella/test': File exists
Error creating outdir ~/test
256 at dir.pl line 7.
The problem is the line of your delete:
[~] $ rm test/ -rf
is wrong. Like most commands, the right syntax would be:
[~] $ <command> <options> <parameters>
so it would be:
[~] $ rm -rf test/